This invention relates to a solid state electrochemical light-emitting device.
Light-emitting devices can be used, for example, in displays (e.g., flat-panel displays), screens (e.g., computer screens), and other items that require illumination. Accordingly, the brightness of the light-emitting device is one important feature of the device. Also, low operating voltages and high efficiencies can improve the viability of producing emissive devices.
Light-emitting devices can release photons in response to excitation of an active component of the device. Emission can be stimulated by applying a voltage across the active component (e.g., an electroluminescent component) of the device. The electroluminescent component can be a polymer, such as a conjugated organic polymer or a polymer containing electroluminescent moieties or layers of organic molecules. Typically, the emission can occur by radiative recombination of an excited charge between layers of a device.
The energy of the emitted light can correspond to the energy difference between bands, i.e., between the ground state and excited state of the materials. The emitted light has an emission profile that includes a maximum emission wavelength, and an emission intensity, measured in luminance (candelas/square meter; cd/m2). The emission profile, and other physical characteristics of the device, can be altered by the electronic structure (e.g., energy gaps) of the material. For example, the brightness, range of color, efficiency, operating voltage, and operating half-lives of light-emitting devices can vary based on the structure of the device.
In general, the invention features a solid state electrochemical light-emitting device having a high maximum luminance, a high external efficiency, a long half-life, and a low operating voltage.
In one aspect, a solid state electrochemical light-emitting device includes a solid layer, a first electrode, and a second electrode. The solid layer includes a non-polymeric metal complex distributed in a polymer matrix. The solid layer has a first surface and a second surface. The first electrode contacts the first surface of the solid layer. The second electrode contacts the second surface of the solid layer.
In another aspect, a solid state light-emitting circuit includes a solid state electrochemical light-emitting device and a driver. The solid state electrochemical light-emitting device includes a solid layer including a metal complex. The solid layer has a first surface and a second surface. A first electrode is in contact with the first surface. A second electrode is in contact with the second surface. The driver includes an AC voltage waveform generator. The driver is configured to apply an AC voltage waveform across first electrode and the second electrode, whereby the solid state electrochemical light emitting device emits light. The driver can also include a DC voltage generator configured to apply a DC voltage across first electrode and the second electrode. The device can have an external efficiency of at least 1.0 percent at a luminance of at least 100 cd/m2.
In yet another aspect, a method of generating light includes applying a light-generating potential across a first electrode and a second electrode of a solid state electrochemical light-emitting device and generating light from the device having a luminance of at least 30 cd/m2. The device includes a solid layer, a first electrode, and a second electrode. The solid layer includes a non-polymeric metal complex distributed in a polymer matrix. The solid layer has a first surface and a second surface. The first electrode contacts the first surface of the solid layer. The second electrode contacts the second surface of the solid layer. The light-generating potential can be applied by applying a DC voltage across the first electrode and the second electrode for a predetermined period of time and applying an AC voltage waveform across the first electrode and the second electrode after the predetermined period of time. The predetermined period of time, known as the charging time, is the time at a voltage at which light visible to the unaided eye is detectable. The AC voltage waveform can be a square wave. The AC voltage waveform can be a 50% duty cycle.
In another aspect, the invention features a method of manufacturing a solid state light-emitting device. The method includes depositing a solid layer including a non-polymeric metal complex distributed in a polymer matrix onto a first electrode and placing a second electrode onto the solid layer. The device can have a luminance of at least 30 cd/m2. The solid layer can be deposited, for example, by spin coating a solution on a surface of the first electrode.
The device is operated and maintained in an inert atmosphere (i.e., in the absence of oxygen and/or water) or is encapsulated in a matrix having low oxygen or water permeability, such as, for example, an epoxy resin. In certain embodiments, the device can have a luminance of at least 30 cd/m2 (e.g., at least 100 cd/m2, at least 200 cd/m2 or at least 1000 cd/m2) at a potential of between 2.5 and 5.0 V. The device can have a time for an intensity of the device to decrease to one-half of a maximum intensity, or half-life, under a 50% duty cycle at 5 V and 1 kHz of at least 200 hours (e.g., at least 300 hours, at least 400 hours, or at least 500 hours). The device can have an external quantum efficiency (xe2x80x9ceqexe2x80x9d) of at least 1.0 percent at a luminance of at least 100 cd/m2, or at least 4.0 percent at a luminance of at least 30 cd/m2.
The non-polymeric metal complex is a metal complex that is not part of a polymer backbone or is not rigidly attached to a polymer chain. For example, the metal complex can be linked to a polymer backbone or other support by a flexible linker, such as a C2-C18 alkylene or oxyalkylene group. The non-polymeric metal complex can be a non-polymeric metal bipyridine complex. The metal can be ruthenium or osmium. The metal complex can be a transition metal complex, such as a hexafluorophosphate salt of a ruthenium bipyridine complex. In certain embodiments, the metal complex is a bipyridine complex. The bipyridine can have one or more hydroxymethyl substituent, a C1-18 alkoxycarbonyl substituent, a C1-18 alkyl substituent, a tert-butyl substituent, or a hydroxy substituent, or combinations thereof.
Each electrode can include indium tin oxide, a metal, such as aluminum, silver, gold, platinum, or palladium, or a conducting polymer, such as polypyrrole. In certain embodiments, the first electrode can be indium tin oxide, aluminum, silver, gold, platinum, or palladium. The second electrode can be indium tin oxide, silver, gold, platinum, palladium, and polypyrrole. For example, the first electrode can be silver and the second electrode can be indium tin oxide.
The polymer matrix can include an organic polymer. The organic polymer can be a glassy polymer and can have a low dielectric constant. The organic polymer can include a polymethylmethacrylate, a polystyrene, or a polycarbonate. The non-polymeric metal complex is distributed in the polymer matrix so that the metal complex is homogeneously dispersed or dissolved in the polymer matrix at least as observed under an optical microscope. The concentration of the metal complex in the polymer matrix can be less than 99 percent, less than 95 percent by volume, less than 80 percent by volume, or less than 70 percent by volume.
The solid state electrochemical light-emitting device based on a solid layer of a metal complex exhibits good shelf life, good operational stability, high efficiency, and high luminance, and operates at low voltages. In addition, device fabrication and operation are straightforward and simple. In fact, low cost fabrication techniques, such as spin coating, can be utilized to make the devices. The device can have an external quantum efficiency of 2-2.5%, and a half-life of over 600 hours. For example, external quantum efficiency can be increased by diluting (Ru(bpy)3)(PF6)2 in a polymer matrix, such as polymethylmethacrylate (PMMA). The half-life can increase when the device is driven with an AC square wave voltage.
For example, the solid state electrochemical light-emitting device based on a Ru(II) complex can be activated to high brightness without elaborate film fabrication or charging schemes, such as solvent-swelling. In addition, reactive cathode materials (e.g., low work function metals such as Ca, Mg) are not needed, and additional electrolyte materials are not added to the solid layer. Swelling the film with solvent is also not needed for the device to operate. Device performance can be increased by improving the quality of the solid layer by, for example, plasma cleaning of the indium tin oxide electrode and by minimizing exposure to air during device fabrication.
By distributing the non-polymeric metal complex in a polymer matrix, the efficiency of the device can be increased by more than a factor of two. For example, a blend comprised of 60 mole % poly(methyl methacrylate) (PMMA)/40 mole % Ru(II) hexafluorophoshphate complex can exhibit an external quantum efficiency greater than 2% at luminance levels in the 100 to 300 cd/m2 range. The increased external quantum efficiency can be maintained without a significant increase in the operating voltage of the device, for example, when the polymer content of the solid layer is less than or equal to 25 percent by volume.
The low operating voltage of the solid state electrochemical light-emitting device allows an applied bias close to the redox potential of the emitter to be used. The low voltage is sufficient to both induce counter-ion migration and effect the required redox reactions to generate light from the solid layer. At slightly higher voltages, charging times decrease dramatically. By applying a high voltage for a brief amount of time, followed by normal operation at low voltage, the charging time of the device can be significantly reduced. Alternative biasing schemes, such as imposing an alternative current field over a direct current bias, can extend device half-life several-fold at comparable brightness. In addition, the increase in device half-life performance achieved through driving with an AC signal is described in U.S. patent application Ser. No. 09/303,101, filed on Apr. 29, 1999, and incorporated herein by reference in its entirety.
By driving the device with an AC voltage, devices first charged with a DC voltage and subsequently driven with an AC voltage can exhibit half-lives of greater than 500 hours, compared to half-lives of about 100 hours when driven with a DC voltage alone. The improved half-lives can be maintained at luminance levels as high as 300 cd/m2. The devices can be implemented as low cost, high efficiency, high brightness backlights for a variety of electronic devices.
Other devices having a multiple layer structure include at least two thin organic layers (i.e., multi-layer devices including a hole transporting layer, and an electron transporting layer) separating the anode and cathode of the device. The multi-layer devices operate like a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Light emitted from such a layered device depends on the direction of the applied bias and the electrode materials. The single solid layer device can emit light under any bias direction when an electrochemically stable electrode material is used, such as a noble metal electrode. In addition, the single solid layer device can employ a variety of electrode materials. Devices prepared with silver as one electrode and indium tin oxide as the second electrode can exhibit external quantum efficiencies in the range of 2.5-5.0% at luminance levels of around 30 cd/m2.
Other features, objects, and advantages of the device and method will be apparent from the description and drawings, and from the claims.